In an optical transmission system, there are cases where the power level of laser light used as a main signal is so high that it may exert a harmful effect on a human body. Therefore, in an optical transmission system, performed is so-called APR (Auto Power Reduction) control, which is the control to suspend outputting of a main signal if disconnection of a transmission line or the like occurs, so as not to emit the main signal into the atmosphere or the like.
Technologies related to the APR control are disclosed, for example, in Patent Documents 1 to 5. Patent Documents 1 to 3 disclose a technology of performing the APR control by detecting the presence or absence of a main signal and a light monitoring signal. However, when the technology of the Patent Documents 1 to 3 is applied to an optical transmission system using a Raman amplifier, as a result of ASE (Amplified Spontaneous Emission) of excitation light used for the Raman amplification, there is a possibility of erroneously detecting the excitation light as the main signal. In such a case, the APR control is not performed.
Alternatively, Patent Document 4 discloses a technology of performing the APR control, in an optical transmission device which comprises a Raman amplifier and receives a main signal, by analyzing a bit pattern of a light monitoring signal and thereby detecting an interruption of the main signal. However, in the case of the technology of Patent Document 4, when the light monitoring signal falls into a communication interruption state owing to disconnection of a transmission line or the like, detection of main signal interruption becomes impossible. Also in such a case, the APR control is not performed.
Still alternatively, Patent Document 5 discloses a technology of monitoring a main signal and an OSC (Optical Supervisory Channel) signal which is supplementary signal light for transmitting a status monitoring signal and a control signal of an optical transmission system, determining the presence or absence of a failure on the basis of the two kinds of signals and, when determining that an abnormality is present, reducing the output of excitation light. By the reduction of the output of excitation light, detection of main signal interruption becomes possible, and the APR control is consequently performed.